Scroll compressors, used as compressors for compressing refrigerant in a refrigerant circuit which executes a refrigerating cycle, have been known in the prior art (for example see Japanese Patent Kokai No. (1993)312156). As shown in FIGS. 6 and 7, such a type of scroll compressor comprises a casing housing therein a fixed and orbiting scrolls (FS, OS) whose involute wraps matingly engage with each other. The fixed scroll (FS) is secured firmly to the casing. The orbiting scroll (OS) is connected to a drive shaft. In this scroll compressor, the orbiting scroll (OS) executes an orbital motion relative to the fixed scroll (FS) by rotation of the drive shaft. The volume of a compression chamber defined between the wraps varies, and the suction, compression, and discharge of refrigerant are carried out repeatedly.
Incidentally, the orbiting scroll (OS) receives a thrust load PS which is an axial force and a radial load PT which is a radial force, when refrigerant is compressed (see FIG. 6). To cope with this, the scroll compressor employs a construction in which a high-level pressure part (P) is provided to apply a high-level refrigerant pressure onto the back surface (lower surface) of the orbiting scroll (OS), whereby the orbiting scroll (OS) is pressed against the fixed scroll (FS) in opposition to the axial force PS by that high-level pressure.
In such an arrangement, if a pressing force PA of the orbiting scroll (OS) is small, and if the vector of a resultant force acting on the orbiting scroll (OS) passes outside the outer periphery of a thrust bearing, the orbiting scroll (OS) is inclined or overturned by the action of a so-called upsetting moment. As a result, there occurs refrigerant leakage, thereby resulting in a decrease in efficiency. By contrast to this, if the pressing force of the orbiting scroll (OS) is greatened, and if the vector of a resultant force acting on the orbiting scroll (OS) is made to pass inside the outer periphery of the thrust bearing, this makes it possible to prevent the orbiting scroll (OS) from overturning.
On the other hand, if there is a change in operating condition of a refrigerating apparatus employing a scroll compressor of the foregoing type thereby causing a variation in high- or low-level pressure, this causes the difference between high-level pressure and low-level pressure (hereinafter the high-low pressure difference) to vary. Consequently, the pressing force PA by the refrigerant pressure of the back surface of the orbiting scroll (OS) varies extensively, particularly with the change in high-level pressure, resulting in an excess or deficiency of the pressing force PA.
In other words, if the area of the high pressure part (P) by which a high-level pressure acts on the orbiting scroll (OS) is such set that the orbiting scroll (OS) does not overturn in the condition in which the high-low pressure difference is great, this leads to deficiency in pressing force because the high-level pressure decreases for example when the high-low pressure difference is small. As a result, the orbiting scroll (OS) is likely to overturn. On the other hand, conversely, if the area of the high pressure part (P) is set according to the condition in which the high-low pressure difference is small, the pressing force of the orbiting scroll (OS) against the fixed scroll (FS) becomes excessive with respect to a minimum required pressing force, for example when the high-low pressure difference becomes great because the high-level pressure increases. As a result, a great thrust force acts on the orbiting scroll (OS) in an upward direction. Accordingly, mechanical loss increases and there is a drop in efficiency.